1. Field of the Invention
The present invention relates to a circuit device and related method for a display device, and more particularly, to a driving signal generating device for staggering transition time of driving signals to prevent image crosstalk for a display device.
2. Description of the Prior Art
With vigorous growth in electronic industrials, a liquid crystal display (LCD) has been widely used in various application fields and the market demand thereof has been increasing rapidly. The operating principle of the LCD is that the LCD utilizes different voltages to drive liquid crystals to arrange in different aspects so as to control the light penetrating amount for each pixel of an image. According to driving manners, the LCD can be divided into three types: Static, Simple Matrix and Active Matrix. The simple matrix type is called a passive type as well, having two models of twisted nematic (TN) and super twisted nematic (STN). The passive TN and STN LCDs both utilize field voltages to drive the liquid crystals so that response times corresponding to central pixels of the panel may become longer with increase of the panel size, affecting image quality. In addition, the TN and STN LCDs have simple architecture and thus are applied to small-size, low-resolution applications, such as electronic dictionaries, mobile phones, personal digital assistants (PDAs), and electronic manometers.
Please refer to FIG. 1, which is a block schematic diagram of a passive LCD 10 according to the prior art. The LCD 10 includes a driving signal generating device 12, a panel 14, a segment electrode driver 16 and a common electrode driver 18. The panel 14 includes segment electrodes SEG1-SEGN and common electrodes COM1-COMK to form multiple intersections each representing a pixel of an image. The operating principle of the passive LCD 10 to drive the panel 14 is that the segment electrode driver 16 transfers driving signals with respect to pixel data to the segment electrodes SEG1-SEGN and moreover cooperates with the timing that the common electrode driver 18 switches on the common electrode. The liquid crystals of each pixel in response to the driving signals twist to certain expected angles and thereby different light amounts are distributed for each pixel according to image data. An image can thus be displayed on the panel 14. In general, an image is displayed, line-by-line, on the screen of the LCD 10. That is, the common electrode driver 18 switches on the common electrode COM1-COMK in sequence, whereas the segment electrode driver 16 accordingly transfers driving signals for pixels of each common electrode. The driving signal generating device 12 is utilized to generate the driving signals for the segment electrode driver 16.
Please refer to FIG. 2, which is a block diagram of the driving signal generating device 12 according to FIG. 1. The driving signal generating device 12 includes a waveform generator 200, a gamma table generator 210, a memory 220 and a multiplexer 230. The waveform generator 200 generates multiple step grayscale waveforms according to a clock signal. For instance, assuming that the display device 10 has thirty gray scales, the waveform generator 200 will be able to generate thirty step grayscale waveforms each corresponding to one of the thirty gray scales. The gamma table generator 210 determines widths of the step grayscale waveforms according to a gamma setting signal SGM, so as to generate grayscale waveforms GW1-GW30. The grayscale waveforms GW1-GW30 are driving signals with different waveform widths, each corresponding to a grayscale value. The memory 220 stores pixel data of each image. The multiplexer 230 selects one of the grayscale waveforms GW1-GW30 for every pixel according to a control signal SSD generated by the memory 220, and outputs the selected grayscale waveform to the segment electrode driver 16. Via the control signal SSD, the driving signal generating device 12 is allowed to transform a pixel value to a grayscale waveform.
Since the liquid crystals may lose flexibility of polarization under long-term driving by same voltage, the display device requires voltage signals having different polarities to drive the liquid crystals when an image is displayed in continuous frames. Please refer to FIG. 3 and FIG. 4, which are signal waveforms of partial pixels of the display device 10 in frames F1-F4. As can be seen from FIGS. 3 and 4, an image includes the frames F1-F4 for use with driving voltage having positive and negative polarities, labeled with + and −. Four pixels are intersections of the segment electrode SEG1 and the common electrodes COM1-COM4, whereas the other four pixels are intersections of the segment electrode SEG2 and the common electrodes COM1-COM4. Assume that the eight pixels have the same grayscale value, and the corresponding grayscale waveforms thereof are one of the grayscale waveforms GW1-GW30. Thus, the pixels corresponding to the segment electrodes SEG1 and SEG2 are corresponding to the same grayscale waveform in each frame. For example, in the frame F1, the grayscale waveforms of the pixel (SEG1 versus COM1) and the pixel (SEG2 versus COM1) fall from high to low (falling time) at the same time and rise from low to high (rising time) at the same time as well.
If the grayscale waveforms of the neighboring pixels have identical transition time (falling time or rising time), crosstalk happens between the neighboring segment electrodes. The grayscale waveforms of the neighboring segment electrodes interact with each other, resulting in inaccurate display of grayscale values and line effects of images. Moreover, the grayscale waveforms are not perfect in implementation and thereby react to transitions (rising or falling) with response times. Therefore, the neighboring segment electrodes will simultaneously demand transition current if the neighboring pixels have the same grayscale value. This challenges system circuits and due to huge workload, the response time may be extended, increasing root-mean-square (RMS) loss of transition current. On the other hand, supposing that the grayscales of all pixels in FIG. 3 and FIG. 4 are fully black or white, the grayscale waveforms thereof keep at high or low without transitions. In this situation, the fully black or white pixels show stronger grayscale depths than other pixels do due to no current loss, resulting in an unbalanced image.
Therefore, the grayscale waveforms corresponding to the neighboring pixels having the same grayscale value have the same transition time when the prior art display device 10 displays the image. The grayscale waveforms are thus affected by each other such that crosstalk happens between the neighboring segment electrodes, resulting in image distortion.